LangChain is an open-source orchestration framework designed to build applications powered by Large Language Models (LLMs) like GPT-4. It enables developers to connect LLMs with external data sources, manage complex, multi-step workflows, and create context-aware applications. It is primarily used for creating chatbots, document analysis tools, and autonomous agents. so langchain give context to AI/LLM. you can use multiple LLM in your app using langchain.

How user flow works?
Takes question from user, covert it into embedding. find most relative embedding in db. then we find that chunk, so chunk+question we will send to LLM( ex chatgpt). langchain provide us tool to handle all of these in easier way with python or javascript supported.